Investigation On The Measurement Of Optical Parameters Of Materials In The Terahertz Band

Direct THz applications of materials require detailed knowledge of their optical constants in this range.However,the optical constants of materials in the THz range are still not well studied.Due to the fact that the study of terahertz sources and detectors is not well developed,the measurement methods of material properties are not perfect enough,and the optical parameters data in the terahertz band are relatively lacking,which directly affects the development of terahertz technology and applications.With the development of terahertz technology in recent years,developing novel terahertz sources,detectors and the applications of terahertz radar,imaging,spectroscopy and so on highly rely on the complete optical-parameter information of materials.Therefore,the measurement technology of optical parameters in the terahertz band has received more and more attention.Based on this demand,research work on the measurement technology of optical parameters in the terahertz band are carried out in this dissertation.This paper mainly studies three optical-parameter measurement methods:1.Terahertz time-domain spectroscopy（TDS）.Based on the amplitude and phase information of the electric field obtained by TDS,the refractive index and the absorption coefficient of the material can be obtained according to the Fresnel equations.Zinc germanium phosphide（ZGP）crystals grown by two different methods were measured,and the optical parameters of different polarizations in the range of 0.2-3THz were obtained.2.Based on the data of Fourier Transform infrared spectroscopy（FTIR）without phase information,a method of optical parameter extraction based on multi-beam interference was proposed.The optical parameters of ZGP crystals in the range of 0.5-6 THz were measured and compared with the TDS measurement results.The feasibility of the method was verified and the measurement range of the optical parameters was broadened.3.For materials which cannot be measured in transmission mode,such as metals,the theory and method of obtaining the optical parameters by using the reflection spectrum and Kramers-Kronig（K-K）relation were studied.The optical parameters of silicon（Si）calculated with the K-K relation were compared with the optical parameters directly extracted from TDS measurement in the transmission mode,verifying the feasibility of the K-K relation processing method.The optical parameters of aluminum in terahertz band were successfully obtained based on the its reflection spectrum.In addition,we reported the construction of a Michelson interferometer based on a terahertz（THz）gas laser pumped by a CO₂ laser.The spectra at 2.52 THz and 3.11THz was accurately measured with the frequency resolution of around 1 GHz.The measurement results showed that the THz laser was monochromatic and had very narrow linewidth.The measured wavelength was consistent with the nominal value.We also performed velocity measurement for the translation mirror based on this interferometer,demonstrating good accordance with the set values for both fixed and variable velocities.It was concluded that the Michelson interferometer could be used to accurately measure the spectrum of a THz source,and the velocity measurement was also possible with the help of a monochromatic THz source,laying the basis for THz applications like spectroscopy,imaging,etc.